Abstract: Using the astrometry from the ESA's Gaia mission, previous works have shown
that the Milky Way stellar halo is dominated by metal-rich stars on highly
eccentric orbits. To shed light on the nature of this prominent halo component,
we have analysed 28 Galaxy analogues in the Auriga suite of cosmological
hydrodynamics zoom-in simulations. Some three quarters of the Auriga galaxies
contain significant components with high radial velocity anisotropy, beta >
0.6. However, only in one third of the hosts do the high-beta stars contribute
significantly to the accreted stellar halo overall, similar to what is observed
in the Milky Way. For this particular subset we reveal the origin of the
dominant stellar halo component with high metallicity, [Fe/H]~-1, and high
orbital anisotropy, beta>0.8, by tracing their stars back to the epoch of
accretion. It appears that, typically, these stars come from a single dwarf
galaxy with a stellar mass of order of 10^9-10^10 Msol that merged around 6-10
Gyr ago, causing a sharp increase in the halo mass. Our study therefore
establishes a firm link between the excess of radially anisotropic stellar
debris in the Milky Way halo and an ancient head-on collision between the young
Milky Way and a massive dwarf galaxy